1. Field of the Invention
The invention relates to a pentose and glucose fermenting yeast cell, suitable for fermentation of a sugar composition comprising C5 and C6 sugar(s) (such composition is herein also designated as the mixed sugar composition). The mixed sugar composition may originate from ligno-cellulosic material. The invention also relates to a process for the production of fermentation product from the mixed sugar composition using the pentose and glucose fermenting yeast cell.
2. Description of Related Art
Most of the ethanol produced as alternative for fossil fuels is currently from fermentation of corn starch and sugar cane based sucrose. In order to reach the ambitious goals for producing renewable fuels, new technologies are being developed for converting non-food biomass into fermentation products such as ethanol. Saccharomyces cerevisiae is the organism of choice in the ethanol industry, but it cannot utilize five-carbon sugars contained in the hemicellulose component of biomass feedstocks. Hemicellulose can make up to 20-30% of biomass, with xylose and arabinose being the most abundant C5 sugars. Heterologous expression of a xylose isomerase (XI) is an option for enabling yeast cells to metabolize and ferment xylose. Likewise, expression of bacterial genes araA, araB, and araD in S. cerevisiae strains results in utilization and efficient alcoholic fermentation of arabinose.
Fermentation of pentose to ethanol by known pentose-fermenting yeast species occurs slowly and results in low yields relative to fermentation rates and ethanol yields that are obtained with conventional yeasts in glucose fermentations. In order to improve the cost effectiveness of the pentose fermentation, it is necessary to increase the rate of fermentation and the ethanol yields obtained.
S. cerevisiae has an inherent preference for glucose. As a consequence, all current pentose fermenting strains demonstrate sequential utilisation of mixtures of glucose and pentoses or at best the pentose fermentation starts at low glucose concentrations.
WO2008041840 describes an eukaryotic cell expressing nucleotide sequences encoding the ara A, ara B and ara D enzymes whereby the expression of these nucleotide sequences confers on the cell the ability to use L-arabinose and/or convert L-arabinose into L-ribulose, and/or xylulose 5-phosphate and/or into a desired fermentation product such as ethanol. Optionally, the eukaryotic cell is also able to convert xylose into ethanol. The strain IMS0003 (page 46) is able to completely consume within 70 hours, glucose, xylose and arabinose, as shown in FIG. 7. FIG. 7 shows that substantial consumption of xylose and arabinose only starts after 20 hours, i.e. when the glucose has already be completely converted. The consumption of glucose and pentose is sequential.
Raamsdonk L M et al., Yeast 2001; 18; 1023-1003 describe co-consumption of sugars or ethanol and glucose in a Saccharomyces cerevisiae strain deleted in the HXK2 gene. However the strain hardly produced ethanol and grows almost exclusively oxidatively in the presence of abundant glucose.
It would be desirable to provide a yeast strain that can an-aerobically ferment pentose either simultaneous with glucose (co-fermentation of pentose and glucose) and/or faster than is known.